Multi-stage amplifiers have been widely used, for example, for linear regulators or low-dropout (LDO) regulators configured to provide a steady and accurately regulated output voltage. A conventional regulator design requires that the output current load is well defined. However, circuits for LDO regulators need to be stable from no or low load current to a specified maximum load current. This requirement changes significantly the transfer function of LDO circuits and makes it a design challenge to provide a stable supply over a variety of the load conditions for the specified accuracy and power consumption. Furthermore, circuits for a typical linear regulator comprising a multi-stage amplifier structure have multiple internal poles and tend to be unstable when they are used in a closed loop configuration.
A well-known technique to increase stability of multi-stage amplifiers is the Miller compensation. The internal poles, i.e. the dominant and the non-dominant pole, are split due to the presence of a Miller compensation capacitance to achieve good phase margin with minimal overhead, thereby making the regulator operate stably.
FIG. 1 illustrates a typical supply feedback linear regulator using a Miller compensated multi-stage amplifier. The linear regulator 100 comprises a first amplifier stage 101, which may be a differential amplifier stage or differential amplifier (also referred to as error amplifier) at the input, and a second amplifier stage 102 at the output, which can be a single amplifier stage or a cascade of multiple of them and/or buffers. The feedback input 107 of the first amplifier stage 101 receives a fraction of the output voltage Vout determined by the feedback factor 106 normally by applying a resistor divider (not shown). The reference input 108 of the first amplifier stage 101 receives a stable voltage reference Vref and the drive voltage to the second amplifier stage 102 changes by a feedback mechanism, i.e. main feedback loop, in case that the output voltage Vout changes relative to the reference voltage Vref, so that a constant output voltage Vout can be maintained.
At the output, the linear regulator 100 may comprise an output capacitance Co (also referred to as output capacitor or stabilization capacitor or bypass capacitor) 104 parallel to the load 105. The output capacitor 104 is used to stabilize the output voltage Vout subject to a change of the load 105, in particular subject to a transient of the load current Iload. The linear regulator 100 using such a multi-stage amplifier is loaded with a certain current which changes the bandwidth of a last amplifying or buffer stage (e.g. a pass device, not shown) across different operating conditions.
In addition, the linear regulator 100 comprises a Miller capacitor 103 having a capacitance Cmiller coupled between the output of the first amplifier stage 101 (which is connected to the input of the second amplifier stage 102) and the output of the second amplifier stage 102 (which is also the output of the linear regulator 100). According to the circuit arrangement of the linear regulator 101 shown in FIG. 1, the equivalent capacitance seen by the first amplifier stage 101 is therefore the Miller capacitance, Cmiller, multiplied by the gain of all stages across it, i.e. the second amplifier stage 102 as shown herein. The use of Miller compensation capacitor can thus provide the pole splitting capability needed to get a stable system across different load conditions.
In the prior art, Cmiller has a constant value, independent of the load conditions, and the constant value is set by stability considerations at low load currents. However, such a large capacitor is not needed at large load currents where the poles are properly split because the output pole goes to higher frequencies. In order to keep a stable operation of the multi-stage amplifier across various load conditions, the bandwidth at the output of the first amplifier stage has to be maximized also at relatively high load currents.